Vehicular tachometers designed for installation after the sale of the original vehicle are usually tapped into the ignition system to locate an input signal that represents actual engine r.p.m. A convenient location in the past has been to tap into the vehicle alternator because the AC signal produced by the alternator is a logical signal source to drive an electronic tachometer. This signal source has been used almost throughout the last decade until recently when most alternator manufacturers no longer exposed the connectors for the alternator AC output signal but instead buried these connectors within the housing of the alternator making it impossible, or practically impossible, for the aftermarket purchaser to tap into this signal inside the alternator, because disassembly of the alternator by a professional mechanic to obtain this signal source is costwise prohibitive and unwise for the vehicle owner to attempt himself.
There has been at least one contemporary, but not admittedly prior, attempt to solve this problem imposed by modern-day alternator manufacturers, and this has taken the form of a single core-piece pick up coil strapped to the exterior of the alternator by a clamping strap with the coil mounted in a U-shaped bracket without any enclosing housing. This pick up coil, while not requiring the disassembly of the alternator, is difficult to attach to the alternator exterior and vehicle vibration frequently causes shifting of the coil from the primary flux leakage path around the alternator armature laminations. Furthermore, the open style of this pick up coil renders it very susceptible to damage from entry of foreign material which eventually degrades the quality of the signal from the coil.
Tachometer calibration has always been a difficult problem in aftermarket designed tachometer kits because the ratio of input signal to actual engine r.p.m. varies widely from vehicle to vehicle. When this input signal is derived from a pick up coil on the exterior of the alternator armature laminations, the input signal frequency varies not only as a function of the number of field poles on the rotor but also as a function of the ratio of the alternator pulley diameter to the crank pulley diameter. In today's on the road vehicles the alternator poles usually vary between four and sixteen, and crank to alternator pulley ratios vary between one and three so that it can easily be seen that the permutation of alternator pole number and pulley ratio yields a wide variety of engine r.p.m. to input signal frequency ratios that must be accommodated in the design of a market-acceptable aftermarket tachometer kit.
The usual method of calibrating tachometers is simply to provide an adjustable potentiometer in the tachometer driving circuitry (for electrical tachometers) that is used in conjunction with a second speed-sensing instrument such as a stroboscope or another tachometer. The new tachometer is adjusted after installation with the engine running until it reads the same as the other instrument and then checked at several different r.p.m.'s for linearity. This of course usually requires the user have expensive instrumentation, or if not, he must use a professional mechanic for installation.
It has also been proposed that a resonant reed be incorporated in the tachometer, driven during calibration by one of the ignition wires, but this is a costly method and produces high voltage transients unacceptable in today's solid-state tachometer driving circuits.
It is the primary object of the present invention to provide a tachometer kit that can be user-calibrated, and includes an improved alternator leakage flux sensing pick up.